Chapter 9, Problem 1
(a) What is the structure of pearlite? (b) Draw a schematic showing all the appropriate phases. Chapeter 9, Problem 2 1. 2. 3. 4.
Draw Draw the Iron Iron Car Carbon bon pha phase se diag diagram ram Mark Mark the pearli pearlite te alloy alloy on the diagram diagram how how much much carbon carbon is in the allou allou What What is hyper hyper and and hypo hypo eutec eutectoid toid alloy alloy Draw the material material structur structure e of both of them them
Chapter 9, Problem 3
(a) What is an isothermal transformation transformation in the solid state? (b) Draw an isothermal transformation diagram for a plain-carbon eutectoid steel and indicate the various decomposition products on it. How can such a diagram be constructed by a series of experiments? Chapter 9, Problem 4
Draw a continuous-cooling transformation diagram for a eutectoid plain-carbon steel. How does it differ from a eutectoid isothermal transformation diagram for a plain-carbon plain-carbon steel? Chapter 9, Solution 4
As depicted in Fig. above, the continuous-cooling transformation diagram for a eutectoid plain-carbon plain-carbon steel possesses longer time intervals between the start and finish times and lower transformation temperatures relative to the isothermal diagram. Also, there are no transformation lines below about 450°C for austenite-to-pearlite austenite-to-pearlite transformations. transformations.
Chapter 9, Problem 5
Describe the full-annealing heat treatment for a plain-carbon steel. What types of microstructures are produced by full annealing (a ( a) a eutectoid steel and ( b) a hypoeutectoid steel? Chapter 9, Problem 6
Describe the process-annealing process-annealing heat treatment for a plain-carbon hypoeutectoid steel with less than 0.3 percent C.
Chapter 9, Solution 6
Plain-carbon hypoeutectoid steels with less than 0.3 percent C are usually process annealed, or stress relieved, by heating to a temperature below the eutectoid temperature, usually between 550 and 650°C. Chapter 9, Problem 7
What is the normalizing heat treatment for steel and what are some of its purposes? Chapter 9, Problem 8
Describe the tempering process for a plain-carbon steel. Chapter 9, Problem 9
Describe the martempering (marquenching) process for a plain-carbon steel. (b) Draw a cooling curve for a martempered (marquenched) austenitized eutectoid plain-carbon steel by using an IT diagram. (c) What type of microstructure is produced after martempering this steel? (d) What are the advantages of martempering? (e) What type of microstructure is produced after tempering a martempered steel? (f) Why is the term martempering a misnomer? Suggest an improved term. Chapter 9, Solution 9
(a) The martempering or marquenching process for a plain-carbon steel produces a martensitic microstructure and consists of: 1. austenitizing the steel; 2. quenching the steel in oil or molten salt at a temperature just slightly above the M s temperature; 3. holding the steel in the quenching medium temperature for a time period sufficient to provide thermal equilibrium of the steel with the medium, without causing the initiation of the austenite-to-bainite transformation; 4. cooling the steel at a moderate rate to room temperature.
(b) A schematic of the cooling curve for a martempered eutectoid plain-carbon steel is shown in the IT diagram below. (d) The advantages of martempering are the minimization of distortion and cracking of the workpiece. (e) By subsequent tempering, the martempered steel develops a tempered martensite microstructure which provides for higher impact energy
(f) Martempering is a misnomer because the steel is quenched rather than tempered; thus, a more accurate term is marquenching. Chapter 9, Problem 10
(a) Describe the austempering process for a plain-carbon steel. Draw a cooling curve for an austempered
austenitized eutectoid plain-carbon steel by using an IT diagram. (b) What is the microstructure produced after austempering a eutectoid plaincarbon steel? (c) Does an austempered steel need to be tempered? Explain. (d) What are the advantages of the austempering process? (e) Disadvantages? Chapter 9, Solution 10
(a) In the austempering process, the steel is austenitized, then quenched in a molten salt bath at a temperature just above the steel Ms temperature, held isothermally until the austenite-to-bainite transformation is complete, then cooled to room temperature in air. The cooling curve for the austempering of eutectoid plain-carbon steel is shown in Fig. P9.34. (b) After austempering, a eutectoid plain-carbon steel has a bainite microstructure. (c) Subsequent tempering is not necessary since the austempering results in hardness, distortion and impact energy values comparable to or better than those associated with a marquenched and tempered steel.
P9.34 Cooling curve for martempering superimposed on a eutectoid plain-carbon steel IT diagram.
(d) Advantages of austempering include: ductility and impact resistance values which exceed those of a quenched and tempered steel; decreased distortion compared to a quenched steel. (e) The disadvantages are the need for a special molten salt bath and limited applicability to specific steels. Chapter 9, Problem 11
(a) Explain the numbering system used by the AISI and SAE for plain-carbon steels. (b) What is the AISISAE system for designating low-alloy steels? Chapter 9, Solution 11
(a) Plain-carbon steels are designated by the AISI-SAE code using four digits. The first two digits, 10, indicate that the steel is a plain-carbon type. The last two digits indicate the nominal carbon content of the steel in hundredths of a percent. Thus a 1030 steel contains a nominal carbon content of 0.30 percent. (b) For low-alloy steels, the AISI-SAE system uses four-digits to designate the alloy: the first two digits indicate the principal alloying element or group of elements in the steel; the second two digits indicate the carbon content in hundredths of a percent.
Chapter 9, Problem 12
(a) Distinguish between natural aging and artificial aging for a precipitation-hardenable alloy. (b) What is the driving force for the decomposition of a supersaturated solid solution of a precipitation-hardenable alloy? (c) What is the first decomposition product of a precipitation-hardenable alloy in the supersaturated solid-solution condition after aging at a low temperature? (d) What are GP zones? . Chapter 9, Problem 13
(a) What are the cast irons? (b) What is their basic range of composition? (c) What are some of the properties of cast irons that make them important engineering materials? (d) What are some of their applications? (e) What are the four basic types of cast irons? Chapter 9, Solution 13
(a) Cast irons are a family of ferrous alloys intended to be cast into a desired shape rather than worked in the solid state. (b) These alloys typically contain 2 to 4 percent C and 1 to 3 percent Si. Additional alloying elements may also be present to control or vary specific properties. (c) Cast irons are easily melted and highly fluid and do not form undesirable surface films or shrink excessively; consequently, they make excellent casting irons. They also possess a wide range of strength and hardness values and can be alloyed to produce superior wear, abrasion, and wear resistance. In general, they are easy to machine. (d) Their applications include engine cylinder blocks and gear boxes, connecting rods, valve and pump casings, gears, rollers, and pinions. (e) The four basic types of cast irons are white, gray, ductile and malleable. Chapter 9, Problem 14
(a) Describe the as-cast microstructure of unalloyed white cast iron at 100 × . (b) Why does the fractured surface of white cast iron appear “white”? Chapter 9, Solution 14
(a) In the as-cast condition, unalloyed white cast iron at 100 × has a microstructure containing large amounts of white iron carbide within a gray pearlitic matrix. (b) The fractured surface of white cast iron appears “white” or bright because of its shiny crystalline surface. Chapter 9, Problem 15
(a) Describe the microstructure of a class 30 gray cast iron in the as-cast condition at 100 × . (b) Why does the fractured surface of a gray cast iron appear gray? (c) What are some of the applications for gray cast irons? Chapter 9, Solution 15
(a) At 100 × , the microstructure of a class 30 gray cast iron, in the as-cast condition, reveals graphite flakes in a light and dark matrix representing free ferrite and pearlite, respectively. (b) The fractured surface of a gray cast iron appears gray because of the exposed graphite. (c) Applications for gray cast irons include cylinder blocks, cylinder heads, clutch plates, heavy gear boxes and diesel engine castings.
Chapter 9, Problem 16
(a) What are the composition ranges for the carbon and silicon in gray cast iron? (b) Why do gray cast irons have relatively high amounts of silicon? (c) What casting conditions favor the formation of gray cast iron? Chapter 9, Solution 16
a. In gray cast iron, the content of carbon and silicon respectively range from 2.5 to 4.0 percent and from 1.0 to 3.0 percent. b. The relatively high silicon content is necessary to promote the formation of graphite flakes. c. Casting conditions which favor the formation of gray cast iron are moderate cooling rates for pearlitic matrix formation and slow cooling rates for ferritic matrix formation.
Chapter 9, Problem 17
How can a fully ferritic matrix be produced in an as-cast gray iron after it has been cast? Chapter 9, Solution 17
A fully ferritic matrix can be produced in an as-cast gray iron by annealing the iron after it has been cast; the heat treatment allows carbon remaining in the matrix to deposit on the graphite flakes. Chapter 9, Problem 18
(a) What are the composition ranges for the carbon and silicon in ductile cast irons? (b) Describe the microstructure of an as-cast grade 80-55-06 ductile cast iron at 100 × . (c) What causes the bull’s-eye structure? (d) Why are ductile cast irons in general more ductile than gray cast irons? (e) What are some applications for ductile cast irons? Chapter 9, Solution 18
(a) In ductile cast irons, the content of carbon and silicon respectively range from 3.0 to 4.0 percent and from 1.8 to 2.8 percent. (b) At 100 × , the microstructure of an as-cast, grade 80-55-06 ductile cast iron consists of a pearlitic matrix containing black graphite nodules within envelopes of free ferrite, (c) which resemble a bull’s eye structure. (d) Ductile cast irons are, in general, more ductile than gray cast irons because their spherical graphite nodules are surrounded by relatively ductile matrix regions which allow significant deformation without fracture. In contrast, the gray cast irons consist of an interlacing network of graphite flakes which can be fractured easily. (e) Applications for ductile cast irons include valve and pump casings, crankshafts, gears, rollers, pinions and slides. Chapter 9, Problem 19
Why does the graphite form spherical nodules in ductile cast irons instead of graphite flakes as in gray cast irons? Chapter 9, Solution 19
Graphite forms spherical nodules in ductile cast irons because the levels of phosphorus and sulfur are reduced significantly compared to those in gray cast irons; these two alloying elements prevent the formation of nodules and thus promote the formation of graphite flakes. Chapter 9, Problem 20
(a) What are the composition ranges of carbon and silicon in malleable cast irons? (b) Describe the microstructure of a ferritic malleable cast iron (grade M3210) at 100 × . (c) How are malleable cast irons produced? (d) What are some of the property advantages of malleable cast irons? (e) What are some
applications for malleable cast irons? Chapter 9, Solution 20
(a) In malleable cast irons, the content of carbon and silicon respectively range from 2.0 to 2.6 percent and from 1.1 to 1.6 percent. (b) At 100 × , the microstructure of a ferritic malleable cast iron (grade M3210) consists of irregular nodular aggregates of graphite, called temper carbon, in a ferritic matrix. (c) Malleable cast irons produced by subjecting white irons to a two stage heat treatment process. First, during graphitization, the white iron is heated above the eutectoid temperature and held for about 3 to 20 hours to transform the iron carbide in the white iron into temper carbon (graphite) and austenite. Next, the austenite is transformed into a ferritic, pearlitic or martensitic matrix depending on the specific cooling schemes applied. (d) Advantageous properties of malleable cast irons are toughness, moderate strength, uniformity of structure and ease of machining and casting. (e) Applications for malleable cast irons include general engineering service requiring good machinability or tight dimensional tolerances and parts requiring high strength such as connecting rods and universal joint yokes. Chapter 9, Problem 1
Draw time-temperature cooling paths for a 1080 steel on an isothermal transformation diagram that will produce the following microstructures. Start with the steels in the austenitic condition at time = 0 and 850°C. (a) 100 percent martensite, (b) 50 percent martensite and 50 percent coarse pearlite, ( c) 100 percent fine pearlite, (d ) 50 percent martensite and 50 percent upper bainite, ( e) 100 percent upper bainite, and (f ) 100 percent lower bainite. Chapter 9, Solution 21 700 (c)
600 (e)
500 400 300
Error: Reference
MS source 200not M
(f )
found
(d )
(a)
f
1
10
102
(b ) 103
104
10 5
Chapter 9, Problem 22 Thin pieces of 0.3 mm thick hot-rolled strips of 1080 steel are heat-treated in the following ways. Use the IT diagram of Fig. 9.23 and other knowledge to determine the microstructure of the steel samples after each heat treatment. (a) Heat 1 h at 860 °C; water-quench. (b) Heat 1 h at 860 °C; water-quench; reheat 1 h at 350 °C. What is the name of this heat treatment? (c) Heat 1 h at 860 °C; quench in molten salt bath at 700 °C and hold 2 h; water quench. (d ) Heat 1 h at 860 °C; quench in molten salt bath at 260 °C and hold 1 min; air-cool. What is the name of this heat treatment? (e) Heat 1 h at 860 °C; quench in molten salt bath at 350 °C; hold 1 h; air-cool. What is the name of this heat treatment? ( f ) Heat 1 h at 860 °C; water-quench; reheat 1 h at 700 °C.
Chapter 9, Solution 22
Figure 9.23 (a) (b) (c) (d) (e) (f)
martensite tempered martensite, quenching and tempering process coarse pearlite martensite, marquenching process bainite, austempering spheroidite
